Introduction

SanDisk's Ultra Plus SSD is their current mainstream-class SSD. The Ultra Plus is designed to be a low cost, power efficient, lightweight, solid state solution for mainstream consumers. Although the Ultra Plus is designed to be cost effective, SanDisk packed it with the latest technology and superior quality toggle mode MLC NAND.

SanDisk was first to launch a drive equipped with an emulated SLC (Single Level Cell) layer baked into an MLC (Multi Level Cell) NAND flash array. The first SSD to utilize SLC emulation technology is the Ultra Plus. There are currently two other drives on the market that employ SLC emulation technology, SanDisk's Extreme II and Samsung's 840 EVO, both of which are enthusiast-class SSD's.

So, what is SLC emulation and why does it matter? 1 bit per cell NAND (SLC) is the "Good Stuff" because it has vastly superior performance and longevity in comparison to (MLC) 2 bit per cell or (TLC) 3 bit per cell NAND technology. Programming a single bit per cell can be accomplished at a much lower latency than programming multiple bits per cell, and is the reason SLC NAND provides superior performance. SLC NAND is typically rated for 100K P/E (Program/Erase) cycles, whereas MLC/TLC NAND is rated for 0.5K to 5K P/E cycles. The drawback to SLC NAND implementation is cost. SLC NAND is an order of magnitude more expensive to produce than MLC or TLC NAND.

SanDisk came up with a brilliant technology called "nCache". nCache Technology allows a layer of an MLC NAND array to be programmed as single bit per cell. This pseudo SLC layer has lower latency and greater endurance than the rest of the MLC NAND array. nCache Technology is used to accumulate small writes (called segments) at high speed and then flush and consolidate them to the larger MLC section of the NAND Flash memory array there by greatly reducing write amplification. Reducing write amplification is key to an MLC based drives longevity, reliability, and long term performance.

Because SanDisk designed the Ultra Plus for low-cost mainstream user implementation more so than all out performance, SanDisk chose to utilize a low cost, power efficient, high performance Marvell Van Gogh Lite 88SS9175-BJM2 4-channel controller to power the Ultra Plus. SanDisk utilizes their own premium 19nm 400Mb/s interface ABL (All Bit Line) Architecture Toggle Mode 2-bit per cell NAND Flash Memory for the Ultra Plus's NAND array. In essence, the Ultra Plus is an Extreme II with a 4-channel controller and a lower price tag. Having four channels in comparison to eight does reduce all out performance, but surprisingly not by all that much.